Ewald Brückl, Vienna University of Technology (Austria)
Michael Behm, Vienna University of Technology (Austria)
Marek Grad, University of Warsaw (Poland)
Oliver Simeoni, Vienna University of Technology (Austria)
Working Group Celebration 2000, University of Warsaw (Poland)
Working Group Alp 2002, Vienna University of Technology (Austria)
Evaluation of wide angle reflection and refraction data acquired during the CELEBRATION 2000 and ALP 2002 experiments extended considerably our knowledge of the deep structure of the Eastern Alps. The application of stacking and tomographic methods to the whole seismic data set resulted in a 3-dimensional P-wave velocity model of the crust and a new depth map of the Moho-discontinuity. The same methods have also been applied to S-waves yielding valuable information on Poisson ratios of the crust and upper mantle and additional insight into the Moho structure. Detailed information on P-wave velocities and seismic boundaries has been gained along profiles crossing the Eastern Alps in N-S, WNW-ESE, and NE-SW directions by interactive modelling using ray tracing techniques. Furthermore, a density model of the upper crust was derived from the 3-dimensional P-wave velocity model and the analysis of Bouguer gravity and densities of rock samples. We interpret the tectonic evolution of the Eastern Alps on the basis of this geophysical data and geological constraints.
Tectonic provinces neighbouring the Eastern Alps in the north (Bohemian Massif) and south (Istria) are apparently little effected by the Alpine orogenesis. Characteristic structures resolved in these areas are most probably of pre-alpidic age. Thinning and weakening of the European crustal basement below the Molasse basin and the accretionary wedge of the Eastern Alps may date back to the extensional phase of the Penninic Ocean. Collision processes determine the structure of the central part of the Eastern Alps. The boundary between European and Adriatic Moho has been clearly resolved and Moho topography indicates subduction directed to the south. Thickening of the East Alpine crust is a consequence of the East Alpine nappe stack overthrusting Flysch, Molasse, and European basement, thrust-folding along a steep crustal ramp and exhumation of the Penninic Tauern Window, and thickening of the Adriatic lower crust. The �rigid� Adriatic indenter with normal crustal thickness and high seismic velocities corresponds at surface to the Adriatic foreland (Istria). The generation of the Pannonian basin and continuing N-S convergence induced crustal thinning by gravitational extension and lateral extrusion to the east. As a consequence of isostatic compensation a considerable Moho uplift took place, forming the newly interpreted Pannonian crustal fragment. The extensional processes find their expression also in low crustal velocities. P-wave velocities of the lower crust are generally lower than 7 km/s, with one exception. This is the area around the Vienna and the north-western Pannonian Basin. Despite low surface elevation and thick low density sediments Moho depth is normal. A high density lower crust is postulated in order to achieve isostatic equilibrium. The process forming this dense lower crust is under debate.